Respiratory tract infections are a major cause of morbidity and mortality of HIV infected children. In normal children, respiratory tract illnesses are usually caused by common viral pathogens such as RSV, parainfluenza and influenza. The role of common viral pathogens in severe respiratory tract illnesses in children with HIV infection is unclear. The primary objective of this study will be to prospectively compare the clinical spectrum and viral etiology of these respiratory illnesses in infants and children with and those without HIV infection. During the proposed study, we will prospectively follow respiratory illnesses that occur in a birth cohort of 180 children born to HIV infected mothers of which 50 will be expected to acquire HIV infection. An additional age, sex, race and socioeconomically similar cohort of normal children born to non-HIV infected mothers will be simultaneously evaluated as an additional control group. Clinical and laboratory evaluation of HIV infection status of infants born to HIV infected mothers is assessed every 6 months. These laboratory evaluations include measurement of:HIV antibodies by ELISA and Western Blot, lymphocyte culture for HIV isolation and lymphocyte function by mitogen stimulation assays. All infants will be evaluated by a study clinician for febrile upper respiratory tract (URI) or any lower respiratory tract illnesses. A physical exam, 02 saturation measurement and nasopharyngeal (NP) viral culture will be performed on initial presentation and every 3 or 4 days until the illness is resolved. Serum and nasal washes will be collected 2 to 6 months. Secondary bacterial or opportunistic co-infections will be monitored. The spectrum of respiratory illnesses will be characterized with regard to type, severity, duration and secondary bacterial an opportunistic infections. These parameters will be related to the child's HIV infection status and clinical stage of HIV infection as specified by the CDC. The etiology distribution of viral agents according to clinically distinct syndromes (febrile URI, bronchiolitis, etc.) will be compared among children without HIV infection versus those with various stages of HIV infection. Duration and quantity of respiratory viral shedding will be determined for each illness episode and compared to HIV infection status conditional upon age and type of respiratory illnesses. Finally, serum and mucosal antibody responses to culture proven respiratory viral illnesses will be assessed and compared with HIV infection status and clinical stage.